Bugfix/IPv6 compatibility for AuthTktAuthenticationPolicy

I've added tests for IPv6 compatibility in the AuthTktAuthenticationPolicy when `include_ip` is set to True. I have also updated the documentation to add an extra note that it is not suggested on IPv6, and fixed the actual issue in `calculate_digest()` so that if it is called with an IPv6 address it doesn't try to call `encode_ip_timestamp()`.

This came about directly because of issue #831 and would effectively close that issue.

If there any style or code issues that I need to fix, please let me know and I will update my brach and submit an updated pull request.